Fullscreen toggle broken on Mac

[jpreiss]

To reproduce:

• drag audience window from laptop to another screen
• press f key
• window goes fullscreen on laptop instead of other screen

Workaround: use the green 'maximize' button instead.

I believe Pyglet windows are associated to the screen where they were first created. I can't find an easy way to detect when a window is dragged to another screen, to change its association. We could maybe reverse-engineer it using pixel coordinates, but not sure it's worth the effort when a workaround exists.